Dialkyl-2,6-naphthalenedicarboxylates and 2,6-naphthalenedicarboxylic acid are useful monomers for the preparation of high performance polymeric materials. For example, dimethyl-2,6-naphthalenedicarboxylate and 2,6-naphthalenedicarboxylic acid can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate) (PEN). Fibers and film manufactured from PEN have improved strength and superior thermal properties relative to other polyester materials. Films made from PEN demonstrate, for example, superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor. Because of its exceptional properties, PEN is especially suitable for applications such as food and beverage containers, particularly for so-called "hot-fill" food and beverage containers, tire cord, magnetic recording tape and electronic components.
Although the dialkyl-2,6-naphthalenedicarboxylates--particularly dimethyl-2,6-naphthalenedicarboxylate--are suitable monomers for preparing PEN and other polymeric materials, in some commercial-scale operations it is preferable to employ 2,6-naphthalenedicarboxylic acid rather than a dialkyl ester. For example, a polyester manufacturer may have equipment and associated processes available for manufacturing polyesters only from an aromatic dicarboxylic acid. In these circumstances, the diester materials would not be suitable and the use of 2,6-naphthalenedicarboxylic acid would be required. Additionally, it is advantageous to use 2,6-naphthalenedicarboxylic acid in the manufacture of polyesters because the condensation of a diacid with a glycol to form a polyester does not form an alcohol by-product as does the condensation of a diester with a glycol. Polyester manufacturers who use diacids such as 2,6-naphthalenedicarboxylic acid do not, therefore, have to provide for the use or sale of the alcohol by-product.
Methods for preparing 2,6-naphthalenedicarboxylic acid include the bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes. Such processes are disclosed in U.S. Pat. Nos. 3,870,754; 4,950,786 and 4,933,491. The bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes, particularly 2,6-dimethylnaphthalene, produces a crude product containing a variety of impurities such as brominated 2,6-naphthalenedicarboxylic acids, 2-formyl-6-naphthoic acid, 2-naphthoic acid and trimellitic acid. These impurities, particularly 2-formyl-6-naphthoic acid, are difficult to remove from crude 2,6-naphthalenedicarboxylic acid. The 2,6-naphthalenedicarboxylic acid must, however, be purified before it can be polymerized to form polymeric materials.
The purification of 2,6-naphthalenedicarboxylic acid is considerably more difficult than the purification of a dialkyl-2,6-naphthalenedicarboxylate primarily due to the low solubility of 2,6-naphthalenedicarboxylic acid in most ordinary solvents, and to its high melting point. In the aforementioned U.S. Pat. No. 4,933,491, for example, 2,6-naphthalenedicarboxylic acid was purified only after reacting the 2,6-naphthalenedicarboxylic acid with a lower alkanoic anhydride to produce a component that is soluble in excess alkanoic anhydride. The "solubilized" 2,6-naphthalenedicarboxylic acid was optionally treated with one or more purification procedures. Xu et al. (Chemistry of Synthetic High Polymers, Vol. 10, pp. 107-11, 1984, Chemical Abstracts CA 102:185547z) describes the purification of 2,6-naphthalenedicarboxylic acid by routine sublimation, recrystallization or distillation as inefficient and difficult due to the poor solubility of 2,6-naphthalenedicarboxylic acid and also because the impurities present, having similar properties, adhere to each other. U.S. Pat. No. 3,649,680 to McNarney discloses a process for purifying aromatic carboxylic acids wherein a mixture of water and an alkanol are added to an impure carboxylic acid paste, the carboxylic acid is separated from the alkanol/water mixture, and the purified carboxylic acid is subsequently washed with water. U.S. Pat. No. 3,671,578 to Ogata discloses a process for preparing 2,6-naphthalenedicarboxylic acid wherein the monoalkali salt of 2,6-naphthalenedicarboxylic acid is heated in water or a water-containing organic solvent, causing disproportionation thereof into 2,6-naphthalenedicarboxylic acid and the dialkali salt of 2,6-naphthalenedicarboxylic acid. U.S. Pat. No. 3,888,921 to Yamamoto et al., discloses a process for purifying 2,6-naphthalenedicarboxylic wherein an aqueous solution of a dialkali salt of crude 2,6-naphthalenedicarboxylic acid is prepared, then 40 to 97 mole percent of the dialkali salt is precipitated as a monoalkali salt while maintaining the pH of the aqueous solution at a value of not lower than 6.3, and converting the precipitate to 2,6-naphthalenedicarboxylic acid. It is disclosed in the Yamamoto et al. patent that the aqueous solution of the dialkali salt of 2,6-naphthalenedicarboxylic acid can be at a temperature of 60.degree. C.-350.degree. C. in the presence of potassium or sodium hydroxide, and it is disclosed that the solution can be treated with a reducing agent such as hydrogen gas, sodium dithionite, lithium aluminum hydride or sodium borohydride. U.S. Pat. No. 3,781,346 to Norton discloses a process for purifying naphthalene carboxylic acids comprising reacting a solid ammonium salt of the acid with steam at a temperature of from about 200.degree. C. to about 300.degree. C. U.S. Pat. No. 4,794,195 to Hayashi et al. discloses that as it is impossible to purify crude naphthalenedicarboxylic acid to a high purity only by crystallization, and that it is necessary to combine the method of crystallization with other methods such as thermal treatment, oxidative treatment or reductive treatment. However, no specific means for conducting such treatments on 2,6-naphthalenedicarboxylic or other naphthalenedicarboxylic acid is disclosed. USSR Inventor's Certificate No. 486,008, to Kulakov et al. published on Jan. 15, 1976, discloses a method for purifying 2,6-naphthalenedicarboxylic acid by treating impure 2,6-naphthalenedicarboxylic acid having a particle size of 0.05-0.35 mm and containing up to 30% naphthalenemonocarboxylic acid with an aliphatic carboxylic acid at 180.degree.-250.degree. C. This Inventor's Certificate teaches that the 0.05-0.35 mm particle size 2,6-naphthalenedicarboxylic acid is obtained by grinding the 2,6-naphthalenedicarboxylic acid and passing it through a screen with 0.05-0.35 mm holes. Kulakov et al. however, does not teach the preparation of 2,6-naphthalenedicarboxylic acid by the liquid phase oxidation of a 2,6-dialkyl or 2-acryl-6-alkyl naphthalene compound, and the reference does not teach the removal of trimellitic acid from 2,6-naphthalenedicarboxylic acid prepared by such liquid-phase oxidation methods. U.S. Pat. No. 3,584,039 to Meyer discloses a process for preparing fiber-grade terephthalic acid by catalytic hydrogen treatment of dissolved impure terephthalic acid.
In contrast to 2,6-naphthalenedicarboxylic acid, the diesters of 2,6-naphthalenedicarboxylic acid are considerably more soluble than 2,6-naphthalenedicarboxylic acid in ordinary organic solvents such as xylenes and alcohols, and can be purified in the dissolved state. Furthermore, these diesters, particularly the dimethyl ester, are sufficiently volatile so that they can be purified by distillation, Therefore, one potential method for preparing purified 2,6-naphthalenedicarboxylic acid is to convert a purified dialkyl-2,6-naphthalenedicarboxylate to 2,6-naphthalenedicarboxylic acid by reacting the diester with water to hydrolyze the ester bonds and form the free dicarboxylic acid. One such process is disclosed in the aforementioned Xu et al. publication. The process disclosed therein comprises forming purified 2,6-naphthalenedicarboxylic acid by dissolving crude dimethyl-2,6-naphthalenedicarboxylate in a xylene, treating with activated carbon, and then crystallizing purified dimethyl-2,6-naphthalenedicarboxylate. The purified dimethyl-2,6-naphthalenedicarboxylate was subsequently hydrolyzed using a 12% potassium hydroxide solution at reflux conditions, and the solution of hydrolyzed ester was acidified with hydrochloric acid to free purified 2,6-naphthalenedicarboxylic acid. While it is reported that this procedure produces high purity 2,6-naphthalenedicarboxylic acid, the process disclosed would not be desirable for large-scale production, The use of concentrated base to hydrolyze the ester and the required use of an acid to free the salt of 2,6-naphthalenedicarboxylic acid is not economical on a large scale.
Other processes for hydrolyzing dialkyl-2,6-naphthalenedicarboxylates are known. For example, dimethyl-2,6-naphthalenedicarboxylate can be reacted with a molar excess of water in a low temperature, low pressure process, e.g. at 350.degree.-430.degree. F., using an acidic catalyst such as an alkylbenzene sulfonic acid or mineral acid. Such a process produces a 2,6-naphthalenedicarboxylic acid product having a small particle size which is difficult to wash and filter, and that requires large quantities of ethylene glycol to prepare slurries for the manufacture of polyesters such as PEN. Additionally, the hydrolysis reaction is slow under these reaction conditions. In European Patent Application 0432910A, published on Jun. 19, 1991, corresponding to U.S. Pat. No. 5,068,410, a process for hydrolyzing dimethyl-2,6-naphthalenedicarboxylate to 2,6-naphthalenedicarboxylic acid is disclosed wherein an aromatic polycarboxylic acid, for example, pyromellitic acid, trimellitic acid or phthalic acid, is used as a catalyst. It is disclosed therein that these catalysts provide for 2,6-naphthalenedicarboxylic acid with a large particle size, and 2,6-naphthalenedicarboxylic acid having an average particle size as large as 67 microns is described in the examples. It is also taught therein that temperatures in the range of 200.degree.-230.degree. C. may be used and that at temperatures greater than 230.degree. C. the corrosive action of the carboxylic acid is increased and, as a result, corrosion occurs on the surface of the vessel material. The European Patent Application discloses that the concentration of aromatic polycarboxylic acid catalyst may be in the range of 0.2-20% by weight, however, all of the examples in the application utilize an amount of aromatic polycarboxylic acid catalyst that is from 15 to 100 weight percent of the dimethyl-2,6-naphthalenedicarboxylate that is hydrolyzed. In European Patent Application 441347A, a process for hydrolyzing a dialkylester of a naphthalenedicarboxylic acid is disclosed wherein the dialkylester is reacted, in the presence of an esterification catalyst, within a temperature range of 70.degree.-350.degree. C. and in a solvent inclusive of a monocarboxylic acid containing no unsaturated bond group and having a carbon number of 1-10. It is also disclosed that the solvent contains water in addition to the monocarboxylic acid.
All of the aforementioned processes for hydrolyzing dimethyl-2,6-naphthalenedicarboxylate require long reaction times and/or the presence of a catalyst or carboxylic acid solvent to carry out the hydrolysis. The art, therefore, needs an improved process for preparing 2,6-naphthalenedicarboxylic acid from a dialkyl-2,6-naphthalenedicarboxylate, and the present invention provides such an improved process.
Consequently, in one aspect of the present invention, a dialkyl-2,6-naphthalenedicarboxylate is hydrolyzed with water at a temperature of at least about 450.degree. F. and the amount of water present is sufficient to solubilize at least about 10% of the 2,6-naphthalenedicarboxylic acid formed. Under these conditions, the hydrolysis of the dialkyl-2,6-naphthalenedicarboxylate is rapid and, particularly when the temperature of the process is about 500.degree. F. or greater, the product 2,6-naphthalenedicarboxylic acid is in the form of large particles having an average size of about 100 microns or greater thereby making the product highly suitable for filtering, washing and preparing PEN. An advantage of the instant invention is that other materials, such as acidic acid catalysts or monocarboxylic acid solvents, need not be added to the hydrolysis mixture and, consequently, need not be separated from the final product as in the prior art processes. Extra processing steps are therefore eliminated. Additionally, when the hydrolysis process of the instant invention is carried out such that a major portion of the 2,6-naphthalenedicarboxylic acid product is dissolved in the hydrolysis water, the 2,6-naphthalenedicarboxylic acid produced is in the form of large, well-formed crystals that are superior for forming low viscosity slurries of 2,6-naphthalenedicarboxylic acid in ethylene glycol. These slurries are used for preparing PEN.
Processes for hydrolyzing dimethylterephthalate to terephthalic acid are also known. U.S. Pat. No. 3,594,414 to Katzschmann discloses a process for preparing fiber-grade terephthalic acid comprising hydrolyzing dimethylterephthalate at a temperature of from about 180.degree. to 280.degree. C., preferably 200.degree.-250.degree. C., and preferably in the presence of neutral salts such as sodium chloride, potassium chloride and calcium chloride. In U.S. Pat. No. 4,302,595 to Schoengen et al., a process is disclosed for preparing fiber-grade terephthalic acid from intermediate stage crude dimethylterephthalate wherein in one step of the process the crude dimethylterephthalate, having a limited content of intermediate oxidation products, is hydrolyzed in water in at least two stages at a temperature between 140.degree. C. and 350.degree. C., preferably from 240.degree. to 280.degree. C., in the first stage, and 180.degree. to 220.degree. C. in the second stage to produce a reaction mixture containing terephthalic acid. Although a hydrolysis temperature of up to 350.degree. C. is disclosed in the Schoengen et al. patent, the examples provided therein use a temperature of 250.degree. C., and it is taught that hydrolysis temperatures above 300.degree. C. do not ensure economical operation.
In another aspect of the present invention, 2,6-naphthalenedicarboxylic acid is purified without requiring its conversion to a diester. Thus, a purified form of 2,6-naphthalenedicarboxylic acid is produced directly and it can be used to manufacture, for example, polyester materials, or such purification process can be used as a preliminary purification before the 2,6-naphthalenedicarboxylic acid is convened to a dialkyl ester. The use of higher purity 2,6-naphthalenedicarboxylic acid in the preparation of a dialkylester provides for a higher quality dialkyl-2,6-naphthalenedicarboxylate.
The processes of this invention can produce 2,6-naphthalenedicarboxylic acid having a mean particle size of greater than 100 microns, and even greater than 200 microns. Large particle size 2,6-naphthalenedicarboxylic acid is advantageous because it provides for more efficient filtration and washing procedures when the 2,6-naphthalenedicarboxylic acid and is being purified. Additionally, the 2,6-naphthalenedicarboxylic acid produced by the processes of this invention have very low levels of "fine" particles that tend to plug filters and other apparatus used for conducting solid-liquid separations.